1. Field of the Invention
This application relates to interference cancellation, specifically as such interference cancellation applies to wireless telecommunications.
2. Description of the Related Art
When observing a cocktail party, a few characteristics relating to the sound are readily apparent. One is that individual conversations can be heard by the participants of those individual conversations and by those in close proximity thereto. The second is that to a person not taking part in a conversation and also not within close proximity to any individual conversation, the collective noise of the many conversations can be heard, but no individual conversation can be discerned from the general noise. In the first instance, the conversations can be heard because the signal (i.e., voice) to interference ratio is strong within close proximity to the speaker. In the second instance, the signal to interference ratio is low, and therefore interference from multiple conversations masks the individual conversations. Both observations, however, highlight the role that interference can have on sound quality.
The same phenomenon is applicable to wireless communications. Interference degrades performance, and unless the signal to interference (noise) ratio is sufficiently high, no particular signal can be discerned. In simplistic terms, that means the voice on the other end of a cellular telephone call may not be heard by a receiver of the call if the interference is too strong. Accordingly, cellular and other wireless communications systems have been designed to insure that the signal received by the wireless device is sufficiently strong relative to the interference such that the voice (and data) can be heard and understood.
Mobile telecommunication service providers and vendors are constantly striving to improve the quality and performance of cellular telephone communications. One such area of focus is the reduction of the interference caused from either (1) adjacent frequency carriers of a cellular base station, known by those skilled in the art as “adjacent channel interference,” or (2) adjacent cellular base stations operating at the same frequency carrier, known by those skilled in the art as “co-channel interference.” The problem of interference is exacerbated when the cellular system is operating within a tight frequency spectrum, especially and including GSM (Global System for Mobile Communications) with a high traffic load. At the present time, there is nothing implemented in a mobile handset that will cancel or greatly reduce either the adjacent channel or co-channel interference. Accordingly, communications utilizing such handsets are not able to optimize the use of the available frequency spectrum.
As will be appreciated by those skilled in the art, similar to the so-called cocktail party problem, the interference observed by a cellular telephone receiver is a summation of individual interference from each of a variety of sources. Such sources of interference include, among other things, adjacent channels within a base station and co-channel and adjacent channels from surrounding base stations, and may include interference from other sources as well. Because nothing is known about the individual signals causing the interference, it is difficult to extract the original voice signal; hence, the cocktail party problem arises.
There are known techniques for reducing or canceling the noise in certain circumstances. For example, in analyzing the summation of multiple interference signals, it is possible to determine whether one particular interference source is dominant over the others contributing to the summation. The term “dominant interference ratio” or DIR, is used to describe the ratio of interference caused by an individual interference source to the summation of interference from all the interference sources combined. In the case where there is a DIR much greater than one (1), there is typically a dominant interferer. In the case where the DIR is less than or approximately equal to one (1), then there is no dominant interferer.
Manufacturers and others in the telecommunications industry have been addressing the interference problem by designing interference cancellation features into their systems. One way to combat the interference is to use frequency hopping where the interference at a certain frequency will be distributed among multiple frequencies instead of being focused on one frequency. See, for example, Olofsson, “Interference Diversity Gain in Frequency Hopping GSM,” 1995 IEEE, 45th Vehicular Technology Conference, Vol. 1, 25-28 Jul. 1995, pages 102-106. Such interference averaging schemes are marginally helpful but do not quite reduce or cancel the interference source to acceptable levels. Another example of interference mitigation is to use a technique known in the art as “Interference Rejection Combining” (IRC). See, for example, Craig et al., “A System Performance Evaluation of 2-Branch Interference Rejection Combining,” 2002 IEEE 56th Vehicular Technology Conference, 2002. Proceedings. VTC 2002-Fall. Vol. 3, 24-28 Sep. 2002, pages 1887-1891. Each of the above-cited references is hereby incorporated by reference in its entirety. IRC is marginally effective where there is a dominant interference source, i.e., those systems where the DIR is much greater than one (1). For those situations where there is not a dominant interference source, the interference suppression techniques being developed by those skilled in the art are mostly ineffective, and the benefits realized pale in comparison to the cost of implementation.
In various interference suppression techniques outside of the cellular telephone industry, there exist various methodologies relating generally to interference suppression. However, those prior art techniques are only applicable in a situation where there is a multiple antenna system. Using those known techniques, those skilled in the art will appreciate that if there is a series of N antennas forming a system, interference can be suppressed for all but one (N−1) of those antennas. This is because at least two actual signals must be observed for the known suppression algorithms to be applicable. In the context of a mobile handset in a cellular telephone system, however, there is a single antenna, not multiple antennas, which is experiencing the interference. Therefore, using conventional known techniques, there is no methodology to suppress multiple interferers experienced by the single antenna in a wireless device.
One option considered in the industry would be to add a second antenna in each handset and then apply cancellation techniques to one of those antennas. While this solution is associated with increases cost, weight and complexity, single antenna interference cancellation with moderate interference suppression is a preferred solution.
U.S. patent Application of Meyer et al., Publication No. U.S. 2002/0141437 (the “Meyer Application”) addresses a method for interference suppression for TDMA and/or FDMA transmissions, with an arbitrary number of receive antennas. The Meyer Application discloses a real value modulation technique wherein the real component of a received signal is separated from an imaginary component of the received signal. The measured received signal is phase shifted from the transmit signal due to channel oscillation and other factors. The received signal is then projected back onto the real axis. The methodology assumes signal and part of interference are orthogonal and, as such, the real and imaginary part of the signal can be exploited to cancel the interference. The inventor believes that the methodology described in the Meyer application is sensitive to the actual data comprising the signal, including some of the embedded data signals such as the training sequence. Also, the complexity of the calculations appears to be significantly higher than the complexity of the present invention. Because of this, the technique is unreasonable to implement in a large-scale wireless telecommunication system.
Another method of interference cancellation, involving at least 2 antenna systems, is combining algorithms, including a “switching combining algorithm” wherein one of the signals is ignored at any given point in time, and an “interference ratio combining algorithm” wherein each signal is weighted in accordance with its signal-to-interference ratio. Again, the problem with these types of combining algorithms is that they require the observations to be independent in order to provide reasonable gain. As will be appreciated by those skilled in the art, the ability to receive highly independent receive signals within the small footprint of a handset is a very challenging task.
The inventor has developed a solution to the single and multiple antenna problems with the present invention. While the present invention may utilize any number of existing known algorithms, including the combining algorithms or the real-value modulation set forth above, the methodology of the present invention preferably uses independent component analysis, or ICA, and applies it to the experience of the single antenna wireless device. ICA is known by those skilled in the art as a statistical model where the observed data is expressed as a linear combination of underlying latent variables. In ICA, the latent variables are assumed non-Gaussian and mutually independent, which assumptions apply to a wireless device antenna as well. The task is to find out both the latent variables and the mixing process. The ICA model used isx=As where x=(x1, x2, . . . xn) is the vector of observed random variables and s=(s1, s2, . . . sn) is the vector of statistically independent latent variables called the independent components, and A is an unknown constant mixing matrix. ICA is very closely related to blind source separation (BSS), where a “source” means the original signal, such as the original voice transmission (or the speaker at a cocktail party). Independent component analysis is described in more detail in the literature, for example in Hyvärinen and Oja, “Independent Component Analysis: A Tutorial,” Helsinki University of Technology, Laboratory of Computer and Information Science, April 1999, and Bingham and Hyvärinen, “A Fast Fixed-Point Algorithm for Independent Component Analysis of Complex Valued Signals,” Neural Networks Research Centre, Helsinki University of Technology, 19th January, 2000, each of which is incorporated by reference.
In conjunction with the preferred embodiment of the present invention, the inventor has created new and non-obvious whitening filter which, when implemented in accordance with the parameters set forth herein, further improves performance of the interference cancellation process.
Accordingly, the present invention solves a major problem of interference in the wireless communications industry. The methodology that has been developed may be implemented in a wireless device to suppress interference signals and improve the signal quality of the received signal. The invention permits a substantial reuse of frequencies in wireless communications, thereby increasing the capacity of the network significantly. The methodology of the present invention is applicable to all wireless technologies, including TDMA, CDMA, GSM, EDGE, WCDMA, 802.11 and 802.16, using any of a variety of modulation techniques, including GMSK, QPSK, 8PSK, and OFDM.
The present invention may be embodied in a receiver used in such wireless communications. The receiver preferably implements the present invention in conjunction with a separation algorithm optimized for the particular technology in order to increase the quality of the received signal. Such a receiver would then be useful in a system in which the co-channel interference and adjacent channel interference may be increased in order to obtain significant capacity increases in the network.
The present invention also may be embodied in the uplink stage of n multi-receivers in a Base Station Transceiver Subsystem (BTS). As such, not only could the interference cancellation algorithm be utilized in a conventional manner for n−1 of those multiple receivers, but rather the present invention enables cancellation of interference for all n of such individual receive antennas in the BTS.
While the present invention has been described: in terms of cellular mobile radio telecommunications systems, the invention is applicable across a broad range of applications and devices wherein single or multiple antennas receive signals that are susceptible to various types of interference.
As set forth in the detailed description of the preferred embodiment, including the graphical results set forth therein, the present invention has achieved results far exceeding those which would be reasonably expected by those skilled in the art attempting to solve the interference problem in wireless communications.